The behavior of structures and structural materials under various stress conditions is of utmost importance for safety and efficiency considerations, and such considerations are at the heart of any structural design. It is known to subject structural elements to stress under controlled conditions for the purpose of controlling the stress distribution within the structure or for testing the structure's behavior under stresses. The former group of activities includes such areas as stress relieving while the latter encompasses fatigue testing and the like.
Consider, for example, stress relieving. As is well known, structural elements are subjected to various fabrication steps that tend to set up non-homogeneous stress conditions within the material. For example, cutting tools used during machining procedures introduce large residual stresses in the workpiece; localized heating during welding induces residual stresses, typically at the exposed toe of the welds; bending loads produce work hardening effects, typically near the surface of the materials.
Vibratory stress relieving devices are known. Such devices comprise a large test stand which is excited by means of a motor-driven unbalanced mass to subject the workpiece to a controlled reversing stress cycle. More particularly, the vibration induces stresses in excess of the residual stresses to be relieved. The vibration causes periodic reversal of these stresses, and the magnitude of the vibration is lowered in order to relieve the initial stresses, in a manner analogous to the well-known demagnetizing procedure. As can be readily appreciated, most vibratory stress relieving machinery is large and requires a considerable amount of power for operation.
Most fatigue testing procedures make use of large machines to subject the structural element under test to relatively low frequency oscillating loads at various stress levels. Fatigue life is then defined in terms of the total number of cycles undergone before failure occurs. The procedure is time-consuming in view of the fact that the structural element typically must be subjected to a large number of cycles before failure occurs. For example, subjecting a beam to a complete bending cycle at a rate of once per second would require approximately 28 hours to generate 100,000 cycles. Many tests require in excess of 1,000,000 cycles.
Aside from the cumbersome special machinery required, the above procedures suffer from an inability to reliably generate the controlled stresses at the locations where the controlled stresses would be most effective. For example, in stress relieving, the stresses induced by the vibrations are not necessarily maximized at the random locations of maximum residual stress within the structural element. Similarly, the fatigue testing procedures do not always apply the greatest stresses at the points of inhomogeneity and discontinuity in the crystal lattice which are most likely to fail under actual dynamic loads.
Once a structural element has been incorporated into a larger structure such as a bridge or building, it can, as a general matter, only be hoped that the element was appropriately stress relieved and that its fatigue characteristics were known to be appropriate. There are methods of non-destructively testing structural elements once in place, such as visual, magnetic, ultrasonic, and X-ray inspection. However, such methods tend to discover defects only after they have long passed the potential stage and have elevated themselves to present dangers. While it is also known to non-destructively test elements by stressing them and sensing high frequency acoustic emissions, it is typically very difficult to apply sufficiently high loads to such elements once they are in place within a larger structure.
In spite of the above problems, mercifully few buildings and bridges collapse, although the consequences of such collapse are inevitably catastrophic. Presumably, buildings and bridges are being sufficiently overdesigned that the lack of precise stress control in the fabricating and testing of the structural elements does not represent a safety problem. It does represent an economic waste, however.